Methods to Derive Geographical Differences of UV Irradiances

Abstract

The continuous negative trend in total ozone column measured in the past 20 years has inspired research on its consequences. It is now very likely that severe depletions of ozone observed especially during 1991–1995 over the northern hemisphere will remain a problem at least until the first decades of the next century. There is overwhelming experimental evidence that, all other atmospheric (and ground) variables being constant, decreases in atmospheric ozone result in UV-B increases at the Earth’s surface, in quantitative agreement with predictions of radiative transfer models. However, it must be noted that stratospheric ozone is not the only component that influences UV-B irradiances at the ground level. Tropospheric ozone and aerosols can reduce global UV-B irradiances appreciably, whereas high albedo can enhance global UV-B irradiances. The variablity of clouds and their influcence on UV-B irradiance further complicate the detection of UV-B changes at the ground level. Only a few studies have monitored UV-B over time scales of decades, and these have yielded conflicting results on the magnitude and even the sign of trends. Some studies may have been affected by problems with instrument instability and calibration, and local pollution trends.

The establishment of trends in solar UV radiation is not the only way to investigate the consequences of solar UV radiation. The natural and man-made differences in stratospheric and tropospheric ozone have lead to measurable differences in solar UV radiation at different locations. Clear-sky UV measurements in midlatitudinal locations of the southern hemisphere are significantly larger than in the northern hemisphere, in agreement with the expected differences due to different ozone columns and sun-earth distance. Taking into account all observing conditions including cloudy skies, the erythemally weighted irradiation at a typical mid-latitudinal southern hemispheric site exceeds the UV levels at a corresponding northern hemisphere site by about 40%. During the summer months the maximum as well as the average daily doses at south pole exceed those measured at mid-latitudes in the northern hemisphere. Due to the large differences in latitude, large differences of UV radiation can be also expected within Europe. This paper presents two methods of deriving geographical differences of UV irradiances, one using data from one instrument run at different sites, the other using measurements from different instruments at one site.